How do I reduce mesh complexity?
If you are new to blender, then start with healthier habits before they turn into vices...

One common error new users make is subdividing too early and creating unnecessary dense topology.

Not only does it make editing way more complicated than what it should be, but will push your computer to its limits very quickly, then you will run out of resources for rigging, animating, doing simulations, particles, volumetrics and, finally, for rendering. Keep it real... Computing power is limited, use it wisely.

Start with simple shapes and work with the absolute minimum number of vertices that give you the shape you are after.

Read: common pitfalls of beginning modellers

Given that you've already put yourself on a bind... use (and apply) the decimate modifier, reduce unnecessary geometry with dissolve edges and faces, and try to keep the number of vertices in check.

For elements that are repeated, model only one and use instances, arrays, dupliverts, duplifaces and dupliframes, those require less resources (read: What techniques have been used to repeat geometry or other detail to the infinite horizon?) 

Softer and rounder meshes can be accomplished with smooth shading, multires, smooth and subsurf modifiers. And speaking of subsurf modifiers, avoid using large values. Hardly ever you should need more than a value of 3. Read: Blender render crash when using subsurf modifiers..

If your quest for detail is part of an obsessive compulsive need to detail things, then create a dense mesh, a normal map and use that map to fake the detail on lower poly meshes.

3d is always a fine balance between artistic goals and the realities of the available computing power.

Sorry if I rain on your parade, but you won't be able to create the prequel to the sequel to the reboot of your favorite sci-fi blockbuster movie in 8K on your personal computer... But that doesn't mean that you can't do amazing work in blender and learn and have lots of fun in the meantime! If you maximize and use your resources in a smart way and keep your expectations on a realistic level, your experience will be more enjoyable. 

Three pieces of unrequested advice:

1- Optimize.

2- Don't complicate what shouldn't be complex (optimize)

3- Use detail only in the places where there is no alternative way to cheat (optimize)